Apparatus for working expandable thermoplastic materials

ABSTRACT

A typical compression relief design extruder screw having a helical flight for advancing and working a thermoplastic material is modified to shorten the feed and compression sections and to include a plurality of force-producing components in at least the compression section of the screw such that at least some portion of the components in any one turn of the flight lie in one plane essentially normal to the axis of rotation of the screw and such that the walls of the flight are uninterrupted. This arrangement controls the location of the rupturing or fragmenting of what is referred to as the solid bed and controls the manner in which it is fragmented to provide an extrudate at a die end of the extruder which has a constant temperature and and which is uniform throughout each successive section of the thermoplastic material. Further, corresponding portions of the successive sections of the thermoplastic material at the die end are caused to have substantially identical time-temperature profiles which is particularly advantageous when extruding expandable insulation in minimizing amplitude of variation in the coaxial capacitance and the diameter of the insulated conductor.

This is a division, of application Ser. No. 557,728 filed Mar. 12, 1975,now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the advancing and working of thermoplasticmaterials to produce homogeneous extrudate, and, more particularly, toapparatus for advancing expandable thermoplastic materials successivelythrough feed, compression, relief and metering zones of an extruder forproducing cellular plastic insulation and jacketing materials withfacilities in the compression zone which do not interrupt the helicalflight of the extruder screw for maintaining a substantially uniformthermal history for successive sections of melt at an output end of themetering zone.

2. Description of the Prior Art

In the extrusion art and particularly in the extrusion of thermoplasticmaterials for insulating conductors for communications systems, a desirefor higher line speeds necessitates higher extruder output. The outputrate for an extruder is limited somewhat by the maximum rate at whichextrusion can be performed while obtaining a uniform extrudate at a dieend.

The thermoplastic material begins to melt along the interface with theinner surface of the extruder barrel. Once melting has begun, threedistinct regions are noted in a cross section of a channel formed by ahelical flight of an extruder screw. These are (1) the unmelted plasticor solid bed, (2) a thin melt film between the solid bed and the barrel,and (3) a melt pool where melted material collects.

The term "solid bed" refers to the plastic material prior to atransformation into a substantially less viscous melt material. Thesolid bed generally remains intact up to a point within a compressionsection of the screw where it will rupture. The later the solid-bedbreakup within the compression section, the more desirable is the screwdesign. As portions of the solid bed are broken off from the initialmass, the portions flow downstream of the screw and continue to melt.Then as the helical flight of the extruder screw advances, the flightwipes off the melt and forms a melt pool on the downstream side of eachsection of a channel formed by the turns of the flight.

At some location in the compression section, the solid bed breaks upinto large portions. The location and size of the portions are afunction of screw design and operating conditions. As the solid bedheats up, the plastic is transformed into a very viscous melt surroundedby less viscous melt. The more viscous melt may resist mixing andtransformation into a less viscous form thereby detracting from thehomogeniety and thermal exposure of the mix.

Improved mixing and temperature distribution have been achieved by usingextruders having increased barrel length to diameter ratios. Theevolution of extruder screw designs is discussed in U.S. Pat. No.3,762,693 issued in the names of R. V. DeBoo and C. B. Heard, Jr.,incorporated by reference hereinto. Terms such as "mixing", "dispersing"and "flight diameters" are terms well known in the art and are defined,for example, in U.S. Pat. Nos. 3,530,534 and 3,762,693.

Slotted ring design screws with pins extending into the channel in ametering section to cause a previously broken solid bed to further breakup and thereby increase conduction heat melting are exemplified by U.S.Pat. No. 3,486,193. This design is characterized by a broken flight topermit mounting the pins continuously around the root diameter sectionof the screw within the metering section. This causes undesirable "deadspaces" which tend to cause a backup of the thermoplastic material.

In U.S. Pat. No. 3,487,503, a multiplicity of pins are arrangedcrosswise or lengthwise of the channel in any region such as themetering or compression sections in which the material is received in amolten or plastic condition to achieve efficient mixing of thethermoplastic material within the extruder resulting in greateruniformity in the extrudate. Although some of the pins in any one turnof the flight of the screw lie in a plane which may be perpendicular tothe axis of the screw, other ones of the pins in that turn of the flightlie outside the plane. Pins arranged in this manner have been found tointroduce excessive restrictions to flow thereby necessitating increasesin the RPM of the extruder screw and shear heat which are critical forcontrol of temperature and hence expansion of the blowing agent.

A uniform solid insulation extrudate has been achieved by using a pinarrangement as disclosed in U.S. Pat. No. 3,762,693, referred tohereinbefore, where the metering section of the screw is provided withat least one group of pins, all of the pins in any one group lying in aplane perpendicular to the axis of rotation of the screw. These pinsfacilitate further breakup of solid bed which reaches the meteringsection to provide small portions with increased surface area toincrease the effect of conductive heat upon the plastic material.

While the above arrangements, and particularly the last described, havebeen found suitable for working the usual thermoplastic materials andachieving a uniform extrudate at the die end, problems arise whenextruding cellular insulation. There, unlike normal solid insulation, itis desired that the thermal history, and not just the temperatures ofcorresponding portions of successive sections of the extrudate, becontrolled to insure uniformity of expansion. This will minimizefluctuations in the coaxial capacitance of the insulated conductor(hereinafter referred to as "capacitance") and diameter-over-dielectric(hereinafter referred to as "DOD").

Conventional extruder screws provide undesirably a premature as well asan intermittent rather than a continuous fragmenting of the solid bed ofthermoplastic material. This presents a problem for thermoplasticmaterials having a chemical blowing agent priorly introduced thereinto.The blowing agent in the broken solid bed is not exposed to as high atemperature for as long a period of time as the blowing agent in themelt between consecutive pieces of solid bed. A low temperature historyof the blowing agent in the solid bed will cause decreased percentexpansion. Variations in the percent expansion causes variation,undesirably, in the capacitance and DOD.

Problems encountered in extruding cellular insulation have beenrecognized. In U.S. Pat. No. 3,287,477, spaced portions of the screw areprovided with longitudinal grooves to form "choke" sections. There isstill a need for facilities to control both the location of thesolid-bed breakup of cellular insulation material and the manner inwhich it is broken up.

SUMMARY OF THE INVENTION

An apparatus for of advancing and working thermoplastic materials toprovide a melt for application to an elongated article includesfacilities for a channel extending helically about an axis of revolutionfrom a supply end in a downstream direction to an output end. Thechannel has uninterrupted side boundaries for advancing a thermoplasticmaterial with a portion of the channel decreasing continuously incross-sectional area in the downstream direction. Means compress andheat the thermoplastic material to progressively melt portions ofsuccessive sections of it subjecting the material to forces exerted byforce-producing components within the portion of the channel in whichthe cross-sectional area decreases. Each of the force-producingcomponents in any portion of the channel has a portion thereof lying ina plane which is perpendicular to the axis of revolution. The apparatuscontinuously fragments unmelted portions of successive sections of thethermoplastic material to provide a melt at the output end, thesuccessive sections of which experience substantially identical thermalhistories.

More particularly, extruding expandable thermoplastic materials whilecontrolling the location and manner of solid-bed breakup includesfacilities for at least one expandable thermoplastic material into afirst zone of an extruder screw having a helical flight formed thereon,and facilities for moving the screw rotatably to feed the materialthrough a channel formed by the flight to a second zone where thematerial is compressed and heated while it is simultaneously subjectedto forces exerted by force-producing components spaced about the surfaceof the screw and extending into the channel in the compression zone. Thecomponents in any one turn of the flight lie in a plane normal of theaxis of rotation of the screw, generally directed radially outwardlytherefrom, and are non-intersecting with the flight. The pressure in thethermoplastic material is relieved in a third zone of the screw whilethe material is advanced through the third zone to a fourth zone. Thethermoplastic material is advanced through a fourth metering zone to adie. The relative lengths of the zones are sized to control the breakupof the solid bed to be within the second zone and the pins arepositioned at the location of solid-bed breakup to cause a controlledmanner of breakup.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the present invention will be more readilyunderstood from the following detailed description of specificembodiments thereof when read in conjunction with the accompanyingdrawings, in which:

Fig. 1 is an elevation view, partially in section, of a conventionalcompression relief design extruder screw modified in accordance with theprinciples of this invention;

FIG. 2 is an enlarged fragmentary detail view of a portion of theextruder screw of FIG. 1 and showing a group of pins connected to a coreof the screw in a compression section thereof;

FIG. 3 is an enlarged sectional view of the extruder screw andassociated barrel of FIG. 1 taken along lines 3--3 showing a pluralityof pins directed outwardly radially from a longitudinal axis of thescrew and lying substantially in a plane perpendicular to the axis, theflight of the screw being uninterrupted in the section of the screwcontaining the pins;

FIGS. 4 and 5 are enlarged elevational views in the vicinity of thecompression section and showing a sequence of breaking up of what isreferred to as the solid bed;

FIG. 6 shows a plan view of the helical channel of the extruder screwunwrapped therefrom and showing areas of melt and solid materials;

FIG. 7 shows the unwrapped channel of FIG. 6 with the force-producingcomponents of this invention superimposed thereon and showing the effectof these components; and

FIG. 8 shows embodiments of alternative arrangements of force-producingcomponents.

DETAILED DESCRIPTION

Referring now to FIG. 1, there is shown an extrusion apparatus,designated generally by the numeral 20, which includes a hopper 21 intowhich at least one thermoplastic material in the form of pellets havinga chemical blowing agent dispersed therein is fed. The hopper 21communicates with an extrusion cylinder designated 22. Thermoplasticmaterials are advanced from an inlet or receiving end 23 of the cylinder22 to an outlet or delivery end 24 thereof where the extrudate is formedinto a covering on a cable core (not shown) being advanced continuouslythrough an extruder head (not shown) adjacent the delivery end.

As can best be seen in FIG. 1, the extrusion cylinder includes a barrelor casing 26 having an internal surface of revolution in the form of acylindrical bore 27 of uniform diameter formed therethrough andconnecting the receiving end 23 to the delivery end 24. The extrusioncylinder 22 also includes a flange 28 at the delivery end 24 thereofwhich facilitates the attachment of adapters, dies and other auxiliaryequipment (none of which are shown but which are well known in the art).

In order to advance the thermoplastic material from the hopper 21 to thedelivery end 24 of the extruder 20, an extruder screw, designatedgenerally by the numeral 31, is disposed concentrically within the bore27. The extruder screw 31 includes a core 32, has an upstream end 33thereof adjacent the hopper 21, and a downstream end 34 adjacent thedelivery end 24.

The extruder screw 31 is of a design commonly referred to as acompression relief design. Beginning at the upstream end 33 thereof, theextruder screw 31 includes, successively, a first constant root diametersection 36 of the core 32 referred to as a feed section (see FIG. 1), auniformly increasing root diameter section 37, referred to as acompression section, a uniformly decreasing root diameter section 38,referred to as a compression relief section, and a uniform diameter rootsection 39, commonly referred to as the metering section.

The extrusion screw 31 is manufactured to have a thread or flight 41formed helically about and extending longitudinally along the core 32.The flight 41 is formed to provide a groove or channel 42 formed by theroot diameter surface of the core 32 and facing side walls 43--43 of theflight. The external diameter and pitches of the flight 41 are generallyidentical and constant along the length of the extruder screw 31 from apoint just beyond the entrance end 33 of the screw to the delivery end34 thereof. However, if desired, the pitch of the flight 41 may be madeto decrease slightly from the portion of the screw adjacent thereceiving end 23 of the bore 27 to the delivery end 24 thereof. Theleading face of the flight 41 is substantially perpendicular to the rootdiameter surface of the core 32 to provide for an improved deliveryaction.

The channel 42 formed between the opposing walls of the flight 41 andthe surface of the core 32 is generally rectangular in shape. It shouldbe clear that the area of the channel 42 is constant from the receivingend 33 to the beginning of the compression section 37. Then the area ofthe channel 42 decreases to the compression relief section 38 whereatthe area increases for a short distance, e.g., typically one-half turnand then remains constant throughout the metering section 39. Acompression relief section is not required to practice this invention.In a typical compression relief screw, the compression relief sectionmay extend, for example, one-half turn.

It is known that high output extruders are required for solidinsulation. However, for foam or cellular insulation, the outputrequirement is decreased. If the same extruder is used for cellularinsulation, the plastic material is moved more slowly through theextruder and there is increased time for heating by conduction asopposed to shear heating.

The extruder barrel 22 tends to move the solid bed in a down channeldirection (see FIG. 4). This is resisted by the screw 31 which istapering. At the beginning of the compression section 37, the solid bedis supported by the bottom of the channel 42.

The plastic material is observed to melt at a higher rate than the rateat which the channel area is being reduced in the compression section 37whereas under ideal conditions, the rates should be substantially equal.This unexpectedly causes a melt pool to collect under the solid bedadjacent the downstream end of the compression section 37 (see FIG. 4)adjacent the core 32 and undermines the solid bed. A portion of thesolid bed breaks off and tends to jam adjacent the shallow end ofcompression section 37 (FIG. 5) with melt collecting between the solidbed and the broken off portion.

Premature, intermittent breakup of the solid bed (FIGS. 5 and 6) isundesirable in extruding cellular insulation. If further breakup isdelayed to the metering section 39, the blowing agent in sections of thematerial across the channel and including large fragments will notexperience the same temperature profile in its advance along the channel42 as will the blowing agent in sections which do not include the largefragments. This causes a high amplitude of variation in the capacitanceand DOD.

It is desirable to control the breakup of the solid bed to fragmentcontinuously the unmelted portions of the successive sections of thethermoplastic material at a predetermined location. Then at any pointalong the channel, corresponding portions of successive sections of thethermoplastic material advanced therepast are at the same temperature.The temperature along the channel varies thus imparting to each portionof each successive section a time-temperature profile, or what iscommonly referred to as thermal history. Of course, it should beunderstood that the temperature of portions within the sectiontransverse of the channel can also vary.

Then although the blowing agent in each section or slice of thethermoplastic material across the channel 42 may be exposed to a varyingthermal history because of the distribution of fragments of the solidbed therein (FIG. 7), the thermal history of the blowing agent incorresponding portions of successive sections of the melt at the outputend does not vary. Of course, the melt temperature at the output end issubstantially constant. This results in a cellular insulated conductor(not shown) with generally constant percent expansion.

In extruding cellular insulation, the tapering of the compressionsection 37 is made to correspond with the melting rate by using ashorter feed and compression section 37.

The general shortening of the feed and compression sections 36 and 37,respectively, effectively controls the location of the solid-bed breakupto be desirably in the vicinity of the downstream end of the compressionsection. The extruder screw 31 may be still further modified to controlthe manner in which the solid bed breaks up, e.g., the size and shape ofthe broken off portions. This will further insure a substantial uniformthermal history of successive sections of the extrudate at the outputend of the extruder 20.

It has been found that force-producing components advantageouslypositioned at the controlled location of solid-bed breakup causedesirably the breakup to be in relatively small portions as compared tothe larger portions experienced in the past. A breakup into small chunksimproves the conduction melting thereof because of the larger surfacearea exposed to the melt and advantageously decreases the amplitude ofvariation in capacitance and/or DOD.

The force-producing components may be constructed similar to thosedescribed in the DeBoo-Heard U.S. Pat. No. 3,762,693. However, thecomponents are positioned at the point at which the solid-bed breakup iscontrolled to occur.

The location of the solid-bed breakup can be determined by so-calledcooling experiments as described in "Plastic Extrusion -- Part I" by R.C. Donovan and D. I. Marhsall and "Plastic Extrusion -- Part II" by E.S. Decker, T. S. Dougherty and C. B. Heard published on pages 74-85 (allincorporated by reference hereinto), of the July-October 1971 issue ofThe Western Electric Engineer. A colorant is introduced into thethermoplastic material and the extruder is operated. After a steadystate condition is reached, the operation is discontinued and cooled andthe thermoplastic material from the channel 42 removed as a continuoussheet and cut transversely into slices. The point along the channel atwhich the cross-sectional slice changes color substantially completelyto that of the colorant is essentially the point of solid-bed breakup.

In order to insure that successive sections of the extrudate haveexperienced substantially the same thermal history, at least thecompression section 37 of the extruder screw 31 is provided withfacilities, designated generally by the numeral 46, for subjecting thematerials to a plurality of forces (see FIG. 2). The force-producingcomponents are preferably attached to the screw 31 approximately one tofour turns upstream of the downstream end of the compression section 37.

As can best be seen in FIG. 2, the facilities 46 include a plurality offorce-producing components 47--47 in the form of pins which are mountedindividually in holes 48--48 formed in the core 32 of the extruder screw31 at least along the compression section of the screws. The holes48--48 are formed so that the centers thereof lie substantially in aplane which is perpendicular to a longitudinal axis of rotation of thecore 32. Additionally, the holes 48--48 are formed in the core 32 sothat when the pins 47--47 are mounted in the associated ones of theholes, the pins are directed radially outward from the longitudinal axisof the core 32.

It should be observed that the arrangement of pins 47--47 differs fromsome prior art arrangements in that all of the pins 47--47 in any oneturn of the flight 41 have at least some portion of the axes thereof orof the pins themselves lying in the so-called plane of pins which isperpendicular to the axis of rotation of the screw.

The structural arrangement of the pins 47--47 with respect to the flight41 is established to minimize the "dead spaces" which appear to occur inthe so-called slotted ring designs. In order to accomplish this, theflight 41 of the extruder screw 31 is uninterrupted at least in thatportion of the screw whereat the pins 47--47 are located. The walls43--43 of the flight 41 of the screw 31 are formed by surfaces whichintersect with the plane containing the pins 47--47 such that thesurfaces are continuous through the plane.

Alternatively, pins 49--49 may be arranged in a row crosswise of thechannel as disclosed in U.S. Pat. No. 3,487,503 (see also FIG. 8).However, this disadvantageously reduces significantly the channel area.In the preferred design, at any one point, the channel is reduced onlyby the width of one pin and hence holds the pressure drop to a minimum.Pins arranged transverse of the channel causes a higher pressure drop,thus necessitating a higher screw speed. Excessive screw speed leadsundesirably to excessive shear heat and increases the difficulty incontrolling the melt temperature and hence the degree of expansion ofthe extrudate.

A still further alternative is to determine where the solid bed-meltinterface occurs by the aforementioned cooling experiments and at thatlocation to install a row of the pins 51--51 (see FIG. 8). This wouldinvolve many turns of the screw 31 and require a substantially largernumber of the pins 47--47.

While the above described invention was made in order to overcomeproblems in the extrusion of cellular insulation, it has been foundsurprisingly that the principles of this invention may be usedadvantageously in the extrusion of solid insulation materials. The priorart has generally not used force-producing components in the compressionsection of the screw 31 because the melting of the plastic materialoccurs there.

However, it has been found that the use of the pins 47--47 in thecompression section preferably supplemented by pins 52--52 in themetering section produces a homogeneous extrudate with each sectionhaving substantially the same thermal history. This is particularly truewith respect to high output extruders where because of the rate ofoutput, the use of pins in the metering section 39 alone may not providebreakup into smaller portions early enough to expose the portion toconductive heat as to achieve uniform temperatures of the extrudate atthe die (not shown).

The use of the pins 47--47 in the compression section 37 insures atimely fragmenting of the solid bed and provides ample time for exposureto conductive heat. This overcomes problems of shrinkage and undesiredbubble formation in the extrusion of some materials such aspolypropylene and high density polyethylene.

Of course, the number of pins 47--47, their precise location, diameterand spacing may vary according to a particular application of theextruder 20, the melt temperature, type of plastic shape extruded, typeof materials fed to the extruder, diameter of the screw 31, and othervariables, the number of groups of pins, by the degree of responsivenessof heating and mixing.

The holes 48--48 may be drilled to a diameter requiring press fitting ofthe pins 47--47. The pins 47--47 may be positively anchored in the core33 by, prior to the insertions thereof, placing solder powder and fluxin the associated hole 48 and thereafter pressing the pin into the holeand applying heat to the pin and adjacent core area until bonding hastaken place. The pins 47--47 are ordinarily oversized with respect tolength and the outer end surfaces ground, machined or otherwise trimmedto a contour conformity with the surface of revolution swept by theflight 41. Of course, the pins 47--47 can be connected to the core 32 inany feasible manner which does not otherwise disrupt the cross-sectionalarea of the channel 42. The pins may be cylindrical, and typically arethree-sixteenths inch diameter with the centers of the holes 48--48thereof spaced apart with a gap of at least three-sixteenths inch on acircumferential circle about the core 32. The pins 47--47 extend intothe predetermined path of the thermoplastic materials along the channel42 with the height of the flight 41.

All of the pins 47--47 need only have a portion thereof in theassociated plane. The pins 47--47, instead of lying substantially in theplane with the pins directed radially outward, could projecttransversely out of the plane where it intersects the flight 41. Or thepins 47--47 could be included in the plane but not necessarily bedirected radially outward from the axis of rotation of the core 32. Andfinally, it is within the scope of this invention that theforce-producing components need not be in the form of pins but could bein the form of vanes such as is common in impeller wheels.

It may also be important to the operation of the extruder apparatus 20in a particular application to have a specified ratio of thecircumferential area of the core 32 of the screw 31 between the pins tothe total area of the free ends of the pins 47--47 in any one plane. Theextruder screw 31 which embodies the principles of this invention couldhave the pins 47--47 arranged so that this ratio lies in the range of 0to 1.

Additional planes of pins 47--47 may be used in the metering sectionwith the upstream one of the planes being located one-half turn orone-half pitch downstream of the compression relief section 38 of thescrew 31. Alternatively, the upstream one of the planes isthree-sixteenths inch downstream of the compression relief section. Thedownstream one of the planes is positioned at the downstream end of thescrew 31 with the other two planes spaced uniformly between the othertwo planes.

OPERATION

Thermoplastic material, such as polyethylene, polymerized vinyl chlorideor the like in granular, powder or pellet form with suitable fillersand/or pigments and blowing agent such as azo-di-carbonamide isintroduced into the hopper 21 of the extruder 20. The extrusion screw 31advances the thermoplastic material from left to right, as viewed inFIG. 1, through the channel 42 between the walls of the flight 41.

In one typical arrangement, the extrusion apparatus 20 includes a screw31 having a barrel diameter of 21/2 inches and length of 66 inches. Thefeed section 36 extends for thirteen inches and has a depth of 375 mils.The compression and compression relief sections 37 and 38, respectively,extend for fifteen and approximately two inches respectively and haveminimum depths of 100 mils. Finally, the metering section extends for 36inches and has a uniform depth of 145 mils.

The general direction of the melting material relative to the screw 31is lengthwise of the helical channel 42. For purposes of explanation,the channel 42 may be regarded as having a helical axis extendinglengthwise of the channel midway between adjacent turns of the flight41. In addition to this movement, the material flows transversely and ina curvilinear fashion about the axis. Each minute element of materialtraverses a path which is a helix having convolutions centered about theaxis which is also a helix. This movement is generated by the frictionalengagement of the inner barrel surface 27 with the outer surface of theplastic material. Because of heat transmission at the interface of thescrew flight 41 and the surface of revolution resulting from frictionalheating, or by heating or cooling equipment, a temperature gradientnormally exists which varies outwardly from the axis to the interface.

As the thermoplastic material is advanced into the compression section37, compacting, softening, melting and mixing takes place therein as thecross section of the channel 42 decreases. The material in thecompression section 37 tends to be drawn out with a change in velocity.Also, and as can be seen in FIG. 7 the pins 47--47 fragment continuouslythe solid bed into small portions, unlike the intermittent breakup shownin FIG. 6. This desirably exposes the blowing agent in correspondingportions of successive sections of the melt to substantially the sametemperature profile along the length of the channel and minimizesfluctuations in capacitance and DOD.

As the material is advanced through the plane of pins 47--47 in thecompression section 37, the pins penetrate the material in the channel42 to disrupt the normal cross section currents of the material andcause breakup of the material.

Then when the material enters the compression relief section 38, thematerial tends to be retracted somewhat with accompanying change invelocity. The metering section 39 functions to tend to bring aboutfurther uniformity throughout the material advanced therethrough withrespect to the temperature, composition and coloring.

The pins 52--52 which may be used in the metering section 39 cause amixing of the plastic material and tend to overcome the tendency of themelt to migrate upstream to the leading or pushing face of the flight41. The melt is urged toward the trailing faces of the flight to mix themelt with the solids and achieve a homogeneous extrudate. By using thepins 47--47 in the compression section supplemented by those in themetering section, a high degree of thermal uniformity of the extrudateis obtained.

It should be observed that in the past, achievement of thermaluniformity of an acceptable degree was obtained principally through areduction of the depth of the channel 42 within the metering section 39.This had the unfortunate corollary effect of reducing the deliverycapability of the extruder 20 and was inadequate to break up effectivelythe solid bed to promote uniformity in thermal history of substantiallyall of the extrudate.

It should be realized that an additional benefit of this inventionaccrues in that presently used screws may be modified as to relativesection length changes and to include the pins 47--47. This permits thecontinued use of present investment in plant and at the same time beingable to increase the effectiveness of the present equipment in producingdual insulated conductors and jackets.

The following four examples relate to the production of 22 gauge dualinsulated conductor having a two mil skin of solid high densitypolyethylene extruded over an eight mil wall of 45% expanded highdensity polyethylene (Allowable variation: DOD ± 1 mil, cap. ± 1.5pf/ft).

EXAMPLE I Prior Art Screw

The screw 31 included an 18 inch long feed section 36 having a depth of0.425 inch a 24 inch long compression section 37 having a depth at theshallow end thereof of 0.110 inch and a metering section 39approximately 23 inches at a depth of 0.110 inch. The depth dimensionrefers to the distance from the top of the flight 41 to the bottom ofthe channel 42. The application of the expandable insulation wasaccomplished at a line speed of 3000 fpm, 39 RPM, a head pressure of4200 lbs./sq. in., and barrel temperatures ranging from 325° F. at thebeginning of the feed section to 410° F. at the head. The melttemperature was 433° F. The variation in DOD was found to be ± 0.2 milsfrom nominal and the variation in capacitance was found to be ± 1.5pf/ft.

EXAMPLE II Prior Art Screw

The screw 31 included a 13 inch long feed section 36 having a depth of0.375 inch., a 15 inch long compression section 37 having a depth of0.100 inch at the shallow end, a 1.25 inch long relief section 38 and ametering section 39 approximately 36 inches in length and having a depthof 0.145 inch. The expandable insulation was applied at a line speed of5000 fpm, at 56 RPM screw speed, a head pressure of 5700 lbs./sq. in.and barrel temperatures ranging from 400° F. at the beginning of thefeed section 36 to 400° F. at the head with a melt temperature fo 445°F. The variation in DOD was ± 0.2 mils, and in capacitance, ± 1.5 pf/ftfrom nominal values.

EXAMPLE III Pins in Metering Section

It should be noted that the screw 31 of EXample II was first modifiedwith only a ring of the pins 52--52 in the metering section 39 and nopins in the compression section 37. No significant changes in the DODand capacitance variations over those recorded for Example II wereobserved.

EXAMPLE IV Pins in Compression and Metering Section

The screw 31 of Example II was modified to include a ring of the pins47--47, each 3/16 inch in diameter on 3/8 inch centers approximately251/2 inches from the upstream end of the feed section 36 and a ring ofthe pins 52--52 in the metering section 39 approximately 31 inches fromthe upstream end of the feed section. The pins in the metering section39 also were 3/16 inch in diameter on 3/8 inch centers measuredcircumferentially along the core surface. The expandable insulationlayer was applied at a line speed of 5000 fpm, 59 RPM screw speed, ahead pressure of 5750 lbs./sq. in. and a constant barrel temperature of395° F. and a melt temperature of 440° F. The variations in DOD andcapacitance surprisingly were found to be only ± 0.2 mils and ± 0.4pf/ft, respectively. The height of the pins 47--47 was slightly lessthan the distance from the core surface at the bottom of the channel 42to the top of the flight 41.

The following example relates to the production of 20 gauge aluminuminsulated with a 2 mil skin of solid high density polyethylene over a 10mil covering of high density polyethylene having a 45% expansion.

EXAMPLE V

The screw 31 included a 24.5 inch long feed section 36 having a depth of0.400 inch, a compression section 31.5 inches long and a depth of 0.100inch at the downstream end thereof, a 1.75 inch long compression reliefsection 38 and a metering section 39 having a length of 33.625 inchesand a depth of 0.135 inch. A ring of 3/16 inch diameter pins spacedapart 3/16 inch were connected to the screw 31 at a distance of 42inches from the beginning of the feed section. The expandable insulationwas applied at a line speed of 4000 fpm, a screw speed of 31 RPM andbarrel temperatures ranging from 225° F. in the feed section to 395° F.in the head. The variations in DOD and capacitance were found to be ±0.2 mil and ± 0.5 pf/ft.

It is to be understood that the above-described arrangements are simplyillustrative of the principles of the invention. Other arrangements maybe devised by those skilled in the art which will embody the principlesof the invention and fall within the spirit and scope thereof.

What is claimed is:
 1. An apparatus for extruding expandablethermoplastic materials, which comprises:means for compressing andheating an expandable thermoplastic material to progressively meltportions of successive sections of the expandable thermoplasticmaterial, said means including a core having a channel formed by anuninterrupted helical flight extending about an axis of rotation of thecore in a downstream direction from a supply end to an output end with acompression section of the channel which decreases in cross-sectionalarea continuously in the downstream direction, said compression sectionbeing such as to cause unmelted portions of the expandable thermoplasticmaterial to fragment at a predetermined location along its length; aplurality of pins connected to the core at the predetermined locationwithin the compression section for subjecting the thermoplastic materialto forces with each of the pins having at least some portion thereoflying in a plane which is perpendicular to the axis of rotation and withthe flight being continuous through the plane to cause the fragmentationof the unmelted portions of successive sections of the thermoplasticmaterial to be continuous; and means for moving rotatably the core toadvance the thermoplastic material along the channel to provide a meltat the output end with corresponding portions of successive sectionshaving substantially identical thermal histories.
 2. The apparatus ofclaim 1, which also includes at least one plurality of force-producingcomponents in a portion of the channel downstream of the portion of thechannel along which the cross-sectional area decreases continuously. 3.An apparatus for extruding expandable thermoplastic materials, whichcomprises:a housing having a longitudinally extending cylindrical boreformed therein; an extrusion screw fitting closely within the bore andhaving a channel which extends substantially from one end to the otherend of the screw and which is formed by walls of an uninterrupted flightthat is generated helically about an axis of rotation of the screw withone end of the screw being a receiving end and the other end being adelivery end for advancing a thermoplastic material from the receivingend of the screw to the delivery end thereof and for homogenizing thematerial; the screw being formed successively from the receiving end tothe delivery end with a feed section, a compression section, acompression relief section, and a metering section wherein the relativelengths of the sections and diameter of the screw and the relativechannel depths in the sections of the screw are such that portions ofsuccessive sections of the thermoplastic material are caused toprogressively melt beginning generally at the downstream end of the feedsection and unmelted portions of successive sections of thethermoplastic material are fragmented at a predetermined location withinthe compression section; and a plurality of pins in the channel in thecompression section at the predetermined location and arranged about thecircumference of the screw for subjecting the thermoplastic materials toa plurality of forces with all the pins in any one turn of the channelhaving at least an integral portion thereof disposed in one plane whichis perpendicular to the axis of rotation of the screw and with the wallsof the channel being uninterrupted and continuous through the plane, thecontrol of the location of the fragmentation and the engagement of thethermoplastic material with the pins being effective to cause thefragmentation to be continuous to provide an extrudate, the successivesections of which have experienced substantially identical thermalhistories.